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The Audi Multitronic CVT


by François Dovat

The core of the Audi Multitronic is a variator working with quite conventional conical pulleys of variable effective diameter, but in which the usual steel push-belt has been substituted for a specific link-chain of 37 mm width.

This chain offers a higher torque capacity for a similar packaging space, it has a better efficiency and it allows a smaller scrolling radius of the tracks. It consists of 1025 individual links and 75 pairs of pins made of high-strength steel. Either 13 or 14 links are lined up next to each other. The torque is transferred by the contact between the pulleys flanks and the ends of the chain's pins. The wear of contact surfaces is said to be only 0.1 to 0.2 mm in 300 000 km.

The input torque is detected by a torque sensor which directly and instantaneously adapts the necessary hydraulic clamping pressure, up to 60 bars. Thus the chain clamping force needed for an absence of contact slip is never set to an excessive level.


The hydraulic system is particularly smart. It consists of two independent circuits, including a low pressure one for cooling and another high pressure one for clamping and ratio adjustments. Moreover, pistons of great surface ensure the pressure of the pulleys flanks against the chain whereas the management of the ratio is ensured by pistons of a smaller surface. By this so called "dual piston" principle, the pressure which clamps and tightens the chain is separated of that which controls the ratio. Consequently only small volumes of oil are necessary to alter the ratio because the volume of oil of the large clamping cylinders is simply transferred from one to the other. A faster response to the instructions of the control system is thus obtained and the power needed by the hydraulic pump is reduced.

A twin concentric oil-cooled multi-disk clutch ensures starting, either in forward direction or in reverse. This double clutch is managed electronically and it is particularly progressive, even allowing a creep similar to that of a hydrodynamic converter. For reverse, a planetary gear set inverter driven by one of the multi-plates clutch is used.

The casing is cast out of magnesium, which reduces its weight by 7 kg compared to an aluminum alloy one. To center the variator according to the available space, it was necessary to place a gear set between the engine and the variator. This gear set also allows an easy adaptation to different engines (S.I, diesels) by altering the input ratio. The hydraulic control unit and its integrated pump, as well as the local electronic system, are installed at the back of the transmission.

Audi announces a slightly lowered fuel consumption compared to the manual version. However this last one has a better efficiency, which is obvious because of its higher maximum speed, but the CVT uses the engine better in most of the driving conditions. In ordinary driving conditions the revs are constantly maintained at the point where the specific fuel consumption is minimum in function of the required power - within the limits of the ratio span, of course.

DRIVING

The driving of the car is extremely pleasant. At WOT acceleration, the revs do not go up at once to the engine's nominal power rpm but instead augment somewhat with the car's speed, from 5000 to 6000 rpm between 80 and 140 km/h (V6 spark ignition engine). This programming brings a conventional feeling of acceleration and acoustics while reducing the noise. The program includes also a sequential control simulating 6 distinct ratios, really useful to slow down with the engine braking. Our test car accepted without any brood quite a few racing launches done by hammering down simultaneously on the brake and accelerator pedals in order to launch the engine before to let the brakes loose. The A4 understood very well that her driver wanted the maximum power immediately: she did rev up to 6000 rpm as soon as the lowest ratio of the variator allowed it, with an initial clutch spin (and slight wheels spin also) at about 3000 crankshaft rpm.

The Audi Multitronic CVT was developed in collaboration with the transmission manufacturer Luk. It is available on the Audi A4 and A6 2.0, 1.8T, 2.4 V6, 3.0 V6 of 220 hp and 300 Nm, likewise on the 2.5 V6 turbo-diesel of 163 hp and 310 Nm.

The ratio span is just over 6, with slight differences according to the engine version.

(© François Dovat)
#86

Technical files

Idée & conception © 1999-2011 van Damme Stéphane.


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